1. Field of the Invention
The invention relates to a method and a device for digital triggering of a recording of a reference signal having a superimposed noise signal.
2. Discussion of the Background
A phase-corrected and time-corrected presentation of a measurement signal on a recording device, such as a digital oscilloscope, requires a triggering, which identifies the signal portion of the measurement signal to be presented on the recording device, for example, via a threshold triggering value, and initiates the recording of the measurement signal on the recording device at the triggering time via a triggering signal derived from the latter.
If a noise signal, which corresponds to a mean-free, white noise with the variance σn2 is superimposed on the measurement signal, there is no fixed triggering time ttrigger as in the case of a noise-free measurement signal, but rather, as shown in FIG. 1, a jitter of the triggering time ttrigger with a variance σt2 dependent upon the variance σn2 of the noise signal.
Starting from FIG. 1 and the triggering equation (1) derived from it, with a triggering threshold value ur of ½ and an edge steepness m of the measurement signal, a fixed value corresponding to equation (2) is obtained for the triggering time ttrigger of a noise-free measurement signal:
                              u          r                =                  m          ·                      t            trigger                                              (        1        )                                          t          trigger                =                  1                      2            ·            m                                              (        2        )            
If a noise signal n with the variance σn2 is superimposed on the measurement signal, starting from the triggering equation (3) with a triggering threshold value ur of ½ and an edge steepness m of the measurement signal, a jittering triggering time ttrigger is obtained according to equation (4) with a mean value of and a variance
  1      2    ·    m  and a variance σt2 according to equation (5).
                              u          r                =                              m            ·                          t              trigger                                +          n                                    (        3        )                                          t          trigger                =                              1                          2              ·              m                                -                      n            2                                              (        4        )                                          σ          t          2                =                              σ            n            2                                n            2                                              (        5        )            
It is evident from equation (5) that the variance σt2 in the jitter of the triggering time for a measurement signal with a superimposed noise signal increases with an increasingly flatter leading edge m of the measurement signal. While the variance σt2 in the jitter of the triggering time in the case of a high-frequency measurement signal with a high edge steepness m according to FIG. 2A is comparatively low, the variance σt2 in the jitter of the triggering time in the case of a low-frequency measurement signal with a low edge steepness m according to FIG. 2B is significantly increased and is no longer negligible.
It is evident from Table 1 in FIG. 3, that with a decreasing frequency of the measurement signal and therefore a decreasing edge steepness m and at the same time a decreasing signal-noise ratio (SNR) and therefore an increasing noise-signal component in measurement signal, the variance σt2 in the jitter of the triggering signal and therefore the inaccuracy of the triggering increases.
Document DE 34 18 500 A1 describes a device and a method, in which the disadvantage of jitter in the triggering time and the associated triggering inaccuracy resulting from a noise superimposed on the measurement signal is overcome by converting the level triggering into a time triggering proportional to the latter so that an appropriate threshold value for the time triggering can be set dependent upon the bandwidth of the measurement signal and the bandwidth of the noise signal.
The disadvantage of a realization of this kind is the comparatively high cost in circuit technology (detection of the zero crossing, generation of the sweep voltage, initialization of the sweep voltage and detection of the set-value pass).